The present invention relates to a process for conversion and recovery of kerogen, as oil, from hydrocarbon-containing solids such as oil shale, tar sands and the like, which provides for increased yields of oil products over prior art processes. More specifically, it relates to a process means for separating solids from hydrocarbons using a novel steam stripping method.
The potential reserves of hydrocarbon fuels contained in such sources as oil shale, tar sands, and the like, are known to be very substantial and form a large portion of the known energy reserves of the United States. The estimated reserves contained in the oil shale deposit known as the Green River Formation alone exceed the reserves of liquid hydrocarbons which can be derived from petroleum. As a result of increasing demand for utilization of domestic reserves in meeting liquid fuel requirements, there has been an increased interest in economically recovering liquid hydrocarbons from oil shale, tar sands and the like, on a commercial scale. Various methods of recovery of liquid hydrocarbons from these deposits have been proposed. However, a principal problem with these methods is their high cost, coupled with low recovery of liquid hydrocarbons, which renders the recovered hydrocarbons too expensive to compete with petroleum crudes recovered by more conventional methods.
Kerogen is a solid organic, primarily hydrocarbon, material having a high molecular weight, i.e. greater than about 3,000 grams per mol, which comprises about 10 to about 30 percent by weight of oil shale. The percentage recovery, as oil, of the kerogen originally present in oil shale is low by most methods known in the art. Even the best methods result in relatively high conversion of kerogen to carbonaceous residue and permanent gases, which are of low economic value in comparison with liquid fuels. Accordingly, the present invention provides a method for increasing the yield of liquid product from oil shale.
Many prior art processes rely on retorting of the hydrocarbon-containing solids to effect conversion and recovery of the kerogen as oil. In principle, the retorting of shale and other similar hydrocarbon-containing solids simply comprises heating of said solids to an elevated temperature and recovery of the vapors which are then evolved. However, net yields of kerogen as oil from such processes are typically much lower than the amount of kerogen initially present, due to the destructive distillation retorting process which causes over-cracking of kerogen to permanent gases and carbonaceous residue.
Many other prior art processes seek to increase the conversion and recovery of kerogen, as oil, by thermal solution techniques wherein the hydrocarbon-containing solid is slurried with a solvent and then brought to a temperature, generally below that of retorting, where the kerogen is converted to a soluble material and is extracted. Such processes typically advocate the use of a specified solvent, usually a pure chemical or a mixture of hydrocarbons of specified composition, which is alien to the nature of the kerogen conversion products obtained from the hydrocarbon-containing solids. Solvent recovery and product oil separation is then accomplished by fractionation. Little is specified as to the manner of separation of the spent solids from the solvent/product oil stream. A key factor in such processes, wherein large amounts of solvent are required to extract relatively small amounts of product oil from solids, is the economical recovery of solvent from the residual solids.
The recovery of kerogen as oil by thermal solution techniques wherein the hydrocarbon-containing solid is slurried with a process-derived recycle oil is described most fully in the U.S. Bureau of Mines Bulletin No. 533, entitled "Thermal Solution and Hydrogenation of Green River Oil Shale," by H. B. Jensen et al. (1953) and incorporated herein by reference. The Bureau of Mines process initially slurries an oil shale with a retort-derived recycle oil which had a boiling range between 229.degree. and 341.degree. C. (445.degree. and 645.degree. F.) at 2 mmHg. However, it was found that more of the retort-derived recycle oil cracked to lighter components or was condensed to heavier components than was produced by conversion of the kerogen contained within the shale. In an attempt to rectify this loss of recycle oil, shale-derived recycle oil was passed through a reactor several times in order to produce a "thermally stable" recycle oil. When the "thermally stable" recycle oil was used in the thermal solution process, it was again found that more of the recycle oil was cracked to lighter components or condensed to heavier components than was produced by conversion of the kerogen contained in the oil shale. The process as thus described was abandoned because it could not sustain recycle oil balance, that is, more recycle oil was cracked to lighter range material or was condensed to heavier range material than was produced from the conversion of kerogen to oil products. The process, thus, could not recover sufficient recycle oil to slurry with the hydrocarbon-containing solids to sustain a continuous process. Addition of hydrogen gas to the slurry under thermal solution conditions did not alleviate the problem of recycle oil loss. Other prior art along similar lines also fails to address the problem of achieving sustained recycle oil balance.
Moreover, the prior art processes do not teach an effective means for separating the spent shale solids from the desirable hydrocarbons with which they are slurried. The present invention comprises a novel use of steam stripping to effect this separation.
Prior art steam stripping processes would not be effectively adaptable to an oil shale separation such as that required herein. In ordinary steam strippers, for example, steam distillation columns, the material to be stripped is passed countercurrent to the steam in a reaction vessel, which is sometimes baffled. The steam strips off the stripped material overhead and the residue remains behind, on the baffles if used, to be removed. Such a countercurrent stripper could not be operated effectively in the current process since the solid residue would eventually lose enough slurrying liquid that it would cease to flow giving rise to difficult removal problems, and at the elevated operating temperatures of the process disclosed herein, the hydrocarbons sought to be recovered would coke up, seriously reducing yield and recycle and also hindering physical operation.